Digital receivers use adaptive circuits, such as equalizers, timing and carrier recovery circuits, cross-polarization cancellers, etc., in the regeneration of an incoming data signal. To compensate for changes in the transmission channel and thereby reduce errors in regenerated signals, the adaptive circuitry is oftentimes adjusted by a decision-directed contol circuit. This control circuit provides the appropriate adjustments in response to a derived error signal and other signals in the receiver. The term "decision-directed" denotes the fact that the adjustments provided are affected by decisions made as to the value of the regenerated data signal.
A characteristic of decision-directed control circuits is that their performance is seriously degraded or fails entirely when the regenerated data signal contains a high error rate. This occurs in digital radio transmission, for example, when atmospheric disturbances result in a high error rate and the loss of receiver synchronization. It is known that the operation of a decision-directed control circuit for adjusting a carrier recovery circuit can be made more immune to the effects of high error rates by defining predicted signal value ranges. Examples of such approaches are described in U.S. Pat. Nos. 4,057,762, issued Nov. 8, 1977, to Namiki and in 4,283,682, issued Aug. 11, 1981, to Sifford et al., and U.S. Pat. application Ser. No. 717,288, filed Mar. 28, 1985, in the name of J. Mardirosian and assigned to the same assignee as the present invention. While these disclosed techniques improve the operation of adaptive carrier recovery circuitry in certain system applications, still further improvements are necessary to meet the performance objectives of other system applications. In addition, a technique which would improve the decision-directed control of other adaptive circuitry, such as equalizers, cross-pol cancellers and timing recovery circuitry would be desirable.